A primary goal in the design of gas turbines or turbo-machines is to have higher efficiencies and wider operating ranges. Thus, a substantial effort is made to understand loss mechanisms and their origins in various components of a gas turbine.
A reduction of losses in any component of a gas turbine used for example as an aircraft engine may lead to a higher efficiency of the entire system. Compressor and turbine stages are especially attractive for loss reductions since the principle of work exchange between them results in an over-proportional increase in net power output for a given efficiency improvement in either of them. This fact has lead to detailed investigations of the flow phenomena in blade and vane rows as well as their unsteady interaction. The identification and localization of loss sources in the flow field plays a major role in this process. On the one hand, it may permit blade and end wall modifications to be applied just at the right place. On the other hand, innovative geometries may be checked against their actual performance.
In spite of improvements in the analysis of loss mechanisms based on computer simulations, experimental investigations are still indispensable. Unsteady flow phenomena like an outflow from a rotor in the absolute frame of reference or eddy shedding in the wake of a vane put high demands on an experimental technique.
Suitable probes should be able to follow fluctuating flow variables such as a local instantaneous total temperature or total pressure as they express a change in entropy which is an indicator for loss mechanisms and, finally, the efficiency. In order to obtain precise results having a higher time resolution than the order of magnitude of the “blade passing frequency” and both shorter and smaller than the order of magnitude of a time scale and dimension of turbulences, both a very high temporal and spatial resolution may be required for the measurements. The required sophistication of such measurement systems becomes obvious by considering the fluctuation frequencies of unsteady flow variables in actual test rigs. In order to be able to resolve typical temperature fluctuation frequencies of 50 kHz, measurements may need to be acquired at acquisition rates of 100 kHz or more.